Optical devices for enlarging microfilms or microfiches, often called microfilm viewers, permit to obtain a high ratio magnification of small size written or illustrated material, such as files, catalogs, documents and data lists of all kinds, from their reduced format on microfilm to a readable format. Microfilms or michrofiches are known to considerably reduce the size of the storage requirements for documentation and information of all kinds, while enabling the user rapid access to the information. Most of the microfilm viewers in use today are of the rear projection screen type. Basically, such viewers comprise a lamp with an appropriate reflector and condenser for illuminating the microfilm with a high light intensity, combined with an objective lens enlarging the illuminated portion of the microfilm and projecting an image onto a translucid viewing screen. The image projected on the translucid screen is therefore observed through the translucid screen, with the screen illuminated by the image projected on the other side of the screen. An enlargement to at least 70% of the original size of the material is necessary in order to reproduce the information stored on the microfilm to a readable format. This in turn requires a projection screen of such dimension that the microfilm reading units normally available on the market are in the form of large size table models with corresponding large space requirements. Portable microfilm readers, however, have been developed which, in some cases, are even collapsible. Unfortunately, even the collapsible units are relatively large and heavy, and they require many controls for their operation. Further disadvantages of projection screen microfilm viewers is that ambiant lighting decreases considerably the contrast of the image projected on the projection screen. In addition, the operation of such units is usually dependent on the availability of electrical outlets.
In order to eliminate some of the disadvantages connected with projection screen microfilm viewers, small pocket reading units have been developed which operate according to a magnifying lens principle with illumination of the microfilm by daylight or by artificial light, power for the artificial light being supplied by batteries. However, currently available microfilm readers of this type are monocular and are, as a consequence, limited to providing a magnification generally not exceeding 24X.
The present invention relates to a microfilm reading unit or viewer which operates according to a binocular microscope principle, that is which is provided with an objective lens system, a pair of eyepieces and, in addition for providing a high magnification in spite of the small size of the unit, a plurality of reflecting elements for increasing the path length of the light rays between the object plane and the image plane. Such an arrangement of the optical path permits to obtain at least a 45X magnification. In addition, binocular viewing of the microfilm material is much less tiring for the human eye than monocular viewing, and it provides a far better image. However, practical binocular viewing devices necessitate means for adjusting the interocular distance, especially since the eye separation in humans varies greatly. The range of normal interocular distance is comprised between 55 and 75mm.
German Patent Publication No. 1,098,233 discloses an arrangement for adjusting the interocular distance in binocular optical devices operating according to the microscope principle, i.e., with objective and eyepiece lens systems wherein the light beam trajectory between the object and the image is reflected by means of mirrors and wherein one eyepiece and its optical axis are angularly positionable around the axis of the other eyepiece at an angle approximating that of the human eye natural movement. The device disclosed comprises a swiveling mirror located in the axis of the light beam which is arranged to follow the swiveling motion of the eyepiece at one half the eyepiece angular displacement by means of a gear drive. The image projected by the mirror onto the eyepiece image plane remains stable during adjustment of the interocular distance.
The arrangement disclosed in the above mentioned German Patent for adjusting the interocular distance comprises an adjustment means for the mirrors disposed in the optical path which consists of a so-called "one-arm" lever attached at an end by way of a ball joint to the non-movable tubular housing and at the other end to the rear extension of the movable eyepiece stud also by means of a ball joint. When the eyepiece stud is rotated, the lever is displaced causing a bracket supporting the mirror at its optical center to be also moved such that the mirror is rotated exactly one half the angle of rotation of the eyepiece stud. In order for the bracket to always abut against the co-operating one-arm lever, the mirror mount is provided with an additional bracket to which is attached the end of a tensioned spring. For the purpose of enabling pivoting of the movable deflection mirrors, they are mounted on pivot disks mounted on the tubular housing and around which the eyepiece studs are pivoting. This arrangement, for the adjustment of the interocular distance, therefore requires many high precision parts and results in a complicated structure. More particularly, the relatively expensive ball joints require special mention, and the lever supporting the ball joints must be preloaded against the eyepiece stud by means of a pressure spring. The ball joints, due to high surface loading, are subject to severe wear and tear which after a short period of time reduces considerably the effective operation of the device. In addition, fatigue of the tension pressure spring forming part of the transmission mechanism causes a considerable decrease of the accuracy of operation of the adjusting mechanism. If the spring breaks, the whole adjustment mechanism becomes inoperative. Finally, the adjustment mechanism lacks an adjustment means for adjusting with precision the angle of the pivotable mirrors, initially and as a result of the loss of precision of the original adjustment through use of the apparatus. Consequently, every single part of the adjustment drive mechanism must be manufactured with high precision and finely adjusted and assembled to the other components with very close tolerances such that a precise and correct adjustment of the mirror must be the result of careful assembly of the component parts of the viewer. When the components become worn, for example, and more particularly the ball joint bearings, due to wear and tear, these parts must be replaced completely, in order for the unit to become again operative with precision.
The present invention has for a principal object to provide a simple and effective device for the adjustment of the interocular distance of the eyepieces in a microfilm viewer, which does not present the disadvantages of the precedently described mechanism and which is particularly well suited for incorporation in microfilm readers having a high magnification ratio.
This object is accomplished, according to the present invention, by providing a drive mechanism having a straight line displaceable plate member and by providing at least one mirror and one eyepiece support mount which are pivotable around a common pivot axis, whereby the plate member is caused to move as a result of the angular displacement of the eyepiece mount thus in turn turning the mirror support, the eyepiece mount and the mirror support being connected to the plate member by arm lever lengths causing the mirror to pivot substantially one half of the pivoting angle of the eyepiece.
The mechanism according to the present invention which, in contrast to the prior art devices, also comprises a precise adjustment of the interocular distance by way of the motion of only one of the eyepieces, for example, is provided with a very simple structure and has a long life expectancy with a high practical safety factor. The mechanism of the invention consists principally of only a pair of simple levers with flat member elements which are arranged for all practical purposes in a single plane. Accordingly, the operation of the mechanism of the invention is also very simple, i.e., only a single displaceable plate member is firmly connected through a lever with the eyepiece mount, and is displaced rectilinearly as a result of the pivoting of the eyepiece mount. The plate member, in turn, displaces the second lever adapted to pivot the deflecting mirror one half of the angular displacement of the eyepiece mount.
The particular arrangement of the present invention for connecting the levers with the plate, as compared to the expensive ball joint mounting of the prior art device, is effected simply by providing right angle projections on the end of the levers engaging into appropriate apertures in the plate. In addition, it is not necessary to provide a spring for the proper operation of the adjustment mechanism of the structure according to the present invention as compared to the prior art mechanism. Due to its simple structure, the mechanism of the invention requires a small space for mounting and, more particularly, its flat, robust design makes the unit particularly well suited for incorporation into portable units of small overall dimensions, such as portable high magnification viewers for the reading of microfilms.
The fact that, in the mechanism according to the present invention, the plate member transferring the pivoting motion of the eyepiece mount to a pivoting motion of the mirror support results only in a linear rather than an exactly required turning motion, is of practically no significance. Since, within the normal range of usual variations in human eye separation, the pivoting angle of the eyepiece and the corresponding pivoting half angles of the mirror are relatively small, any misalignment of the image reflected by the mirror is only of such insignificant degree that the resulting shift of the image is practically not recognized by the human eye at all. The misalignment of the image as a result of using a linearly displaceable plate member is particularly small in the embodiment according to a further characteristic of the present invention, wherein the displacement direction of the plate is parallel to the optical axes of the eyepieces when the eyepieces are adjusted to the smallest interocular distance, whereby the eyepiece mount and the mirror support are aligned with the plate member in such a manner that the refelective surface of the mirror, at its middle position, is rotated exactly one half the angular displacement of the eyepiece. In such a case, a mathematically correct image without any misalignment is obtained, assuming that most interocular distances are of normal dimensions. In the direction of the lowest as well as of the highest limit of the interocular distance however, due to a relatively small pivoting angle, as hereinbefore explained, the follow-up of the mirror rotation at one half the angle of pivoting of the eyepiece provides a very good approximation.
Advantageously, an adjustment or calibration screw is provided for the original set up of the angular position of the mirror, such adjustment screw being preferably provided at the end of the mirror support. With such a simple arrangement, the preset calibration screw may be operated from the outside of the housing by means of a screw driver, and the position of the mirror may be adjusted for an average interocular distance such that the images at the two eyepieces match exactly and completely. This pre-adjustment of the mirror is usually necessary only once, namely prior to the first use of the viewer.
A further improvement of the accuracy of the adjustment mechanism for interocular distance results additionally from providing a spring, according to the present invention, mounted between the eyepiece and the mirror support, such as to provide an appropriate preload eliminating any mechanical play in the drive mechanism.
A modification of the present invention provides both eyepieces as pivotable, and by providing a single common guide plate member linearly movable as a result of the relative pivoting of the eyepiece mounts appropriately aligned relative to the plate member, the plate member in turn controls the pivoting of two mirrors such as to coordinate the pivoting of the mirrors with the relative pivoting of the separate eyepieces. In this embodiment of the invention, therefore, both eyepieces are simultaneously pivotable about a common axis, and are provided with a common drive mechanism driven by the pivoting motion of the eyepiece mounts which in turn cause pivoting of the image projected by the mirrors into the plane of the eyepieces. Such a structure is therefore capable of providing perfectly symmetrical images.
The objects and advantages of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawing wherein like reference numerals designate like or equivalent parts and in which: